1. Field of the Invention
The present invention relates to an antilock brake control system for automobiles, and more particularly to a technique which makes possible the control of normally adaptable brake pressure in correspondence to the coefficient of friction between the tires and the road.
2. Prior Art
The antilock brake control apparatus shown in Japanese Patent Application, first publication, laid open number 60-35647, is conventionally known as an antilock brake control apparatus which prevents the locking of the wheels of an automobile when braking.
This antilock brake control apparatus has a determining mechanism, which compares previously set standard wheel speeds and standard wheel accelerations with wheel speeds which are detected by means of a wheel speed sensor or wheel accelerations which are differential values thereof, and determines a .mu.-level (corresponding to the coefficient of friction) between the road surface and the tires, and a changing mechanism, which resets the pressure reduction and stopping level of brake fluid pressure when antilock brake controlling is based on the .mu.-level determined by the determining means.
By means of this, in the case in which the "H" level (the coefficient of friction is large) has been determined by the determining mechanism, the pressure reduction and stopping level is set at a low "H-.mu.", and the reduction in brake pressure is stopped comparatively early. Furthermore, in the case in which the "L" level (the coefficient of friction is small) has been determined by the determining mechanism, the pressure reduction and stopping level is set at a high "L-.mu.", and the reduction in brake pressure is stopped comparatively late.
In an antilock brake control apparatus with the above construction, the .mu.-level is determined based on the wheel speed, the wheel acceleration, the standard wheel speed and the standard wheel acceleration, or is based on a slip ratio calculated from the wheel speed and a simulated body speed (for example, a body speed inferred from fixed deceleration (tendencies)). In addition, it makes the appropriate setting of the pressure reduction and stopping level of the brake pressure to "H-.mu." or "L-.mu.". However, this type of determining and setting method has a problem, as shown by a line in FIG. 6, in that when momentary large slippage occurs as a result of unevennesses in the road surface (for example, the wheels bounce while the brakes are being applied), although the value should be H-.mu., the recovery (reduction) of the slip ratio takes some time, so that "L-.mu." is erroneously determined.
Furthermore, when brake pressure is being decreased, as shown by the line in FIG. 7, when the recovery of the wheel speed is small (the acceleration is small), for example, in the case in which a friction-reducing material such as sand or water is temporarily present on the road surface, or in the case in which the fluid pressure is being controlled in a manner appropriate for a "H-.mu." road surface, or the like, although the actual value is H-.mu., "L-.mu." is erroneously determined.